This invention relates generally to circuit packaging techniques, and more particularly, to techniques for packaging high-frequency amplifiers, such as feed-forward amplifiers used in cable television and other applications. Since the invention is to be described with reference to a high-frequency feed-forward amplifier, an understanding of the principles of operation of the amplifier is necessary by way of background.
In recent years, cable television systems have been required to handle an increasing number of channels. Distortion and cross-modulation between the channels limits the number of amplifiers that can be cascaded and the maximum spacing between amplifiers.
A feed-forward technique is commonly used to reduce the distortion in individual amplifiers by a factor of at least one hundred over relatively wide bandwidths. The basic technique involves the use of two cancellation loops. In a signal cancellation loop, the distorted signal from a main amplifier is subtractively combined with a delayed form of the original signal, to produce a signal containing substantially only error components. In the second cancellation loop, the error signal derived from the signal cancellation loop is amplified, and fed forward to be subtractively combined with the output of the main amplifier. The error components cancel and a distortion-free signal remains.
While the theory of feed-forward error cancellation is relatively simple and well known, a practical device must control signal amplitudes and phase angles to close tolerances in order to achieve error cancellation over a wide frequency band. The cross-referenced application is concerned with an approach to accomplish this objective. Another aspect of the design of such an amplifier is that, like all radio-frequency (rf) circuits, it requires close attention to the manner in which grounding is effected, and it is subject to heating and cross-modulation problems, especially if housed in a small, low-cost package.
Poor grounding, or long paths to ground, can result in spurious amplifier feedback signals. These may vary in strength and polarity across the frequency band in which the amplifier operates, and therefore create an undesirable "ripple" in the gain-frequency characteristic of the amplifier. Accordingly, a key requirement is that there be a reliable grounding structure, with extremely short paths to ground.
Another concern in packaging of rf amplifier circuits is that the various circuit modules have to be adequately isolated from each other and from the environment of the amplifier. A sometimes opposing consideration is that the device package should be as small as possible. The size and construction of the circuit package also play an important role in determining how heat is dissipated from the circuit. Ideally, the high-power components, namely the amplifier modules, should be located as close as possible to a heat sink. Yet another requirement for amplifiers of this type is that certain circuit components that are used for tuning must be conveniently accessible.
Feed-forward amplifiers of the prior art fall into two categories: discrete and hybrid implementations. Amplifiers in the discrete category are conveniently modular, but have poor grounding and poor isolation between components. Tuning is difficult, and it is also difficult to achieve high gains or high bandwidths. Hybrid amplifiers consist of an alumina substrate carrying all of the active and passive components, soldered onto a metal block that serves as both as ground plane and a heat sink. The resulting lack of modularity results in poor component isolation.
It will be appreciated from the foregoing that there is a need for a packaging construction that addresses all of these problems together. Basically, the ideal package for an rf amplifier should have very good grounding properties, heat dissipation properties, and isolation properties. Importantly, these properties must be obtained without high cost, in a package that is convenient to assemble and to effect circuit tuning. The present invention is directed to these ends.